Photocell amplifier



NOV. 14, R, H RANGER 2,179,954

PHOTOOELL AMPLIFIER Filed June 6, 1935 INVENTOR R\CH D .RANGER BY ATTORN EY Patented Nov. 14, 1939 UNITED STATES PHOTO CELL AlVIPLIFIER Richard Howland Ranger, Newark, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 6, 1935, Serial N0. 25,220

3 Claims.

This invention relates to photocell ampliers and, in particular, to an integrating type of amplifier, which has an output proportional to the product of intensity of the impinging light on the photocell and the duration of such impingement. rlhe invention in one of its preferred forms finds particular application in facsimile transmission systems wherein it is desired to transmit half-tone pictures or subjects by the so-called on-oit keying systems. However, the invention is not restricted to the above eld but is also directed toward counting systems wherein it is desired to have an indication of units based on vthe duration of time when light of a predetermined or even variable intensity is either revealed to or masked from a suitable light translating element.

As is well known in the art of picture transmission, over long distance radio circuits or wire communication links or channels, it is extremely desirable to dispense with amplitude modulation and resort to on-oif keying, i. e. telegraphic keying. in order to give the effect of half-tones with such a system, it then becomes necessary to resort to some means of time modulation to take the place of amplitude modulation to represent the various tone values of the elemental areas making up the pictures. Thus, to represent in the transmitted signal elemental areas of the subject which are of great density, for example, black or approaching black, the signalling impulses are sent at relatively 10W frequency, but where the subject matter has a density varying in the direction of white, as for example, between light gray and white, the frequency of the pulses becomes correspondingly greater the more closely complete white is approached. The marking impulses are generally made equal in duration for all values of density, but the spacing of the impulses is made to vary so that time modulation which is introduced by the spacing interval between adjacent signalling impulses controls the transmission. This process can readily be reversed so as to transmit negatives, in which case the marking impulses are made variable with respect to time and all spacing intervals are made equal. 1n this case, some means of reversing the received signal must be used if a positive picture at the receiver is desired. Such reversing means are Well known to the art, but for illustration of the principles of the invention it may be assumed tc be accomplished through the use of an extra stage of D. C. amplification, which, as is well known, reverses the polarity of the signal.

It is, accordingly, one object of this invention,

to furnish a photoelectric cell amplifier for use in a facsimile system where it is desired to use on-off keying when transmitting half-tone material.

It is a further object of this invention to provide an amplier for a photoelectric cell which will have an output in accordance with the integrated value of impinging light and time duration of said impingement.

Further objects and advantages of this invention will become evident upon reading the following specication in conjunction with the accompanying drawing wherein the single ligure thereof illustrates schematically a photoelectric cell amplifier designed in accordance with this invention.

Referring to the drawing, cooperating with the photosensitive device I, which may be any conventional type photoelectric cell, is an optical system 5l designed to pick up the light reflected from each elemental area of the subject 53. Connected in series with the electrodes of the photocell l is a battery 3 and a condenser 5. When light reflected from the subject 53 falls on the photocell I, current starts to flow through the condenser 5 and begins to build up a charge therein. A rst amplifying tube il has its grid i5 and cathode l@ connected so as to amplify the resulting output from the photocell I and in the grid lead a biasing source S is provided.

As the condenser 5 is charged by light falling in the cell l, the charge, due to the indicated polarity of the battery 3, will be positive at the point of connection of the condenser 5 and the battery 9, so that the grid I5 of the tube II becomes positive with respect to cathode I E. The biasing source for tube II, indicated by the battery fl serves to maintain the grid I5 at cut-od potential in the absence of any charge on the condenser 5. As the charge builds up across the condenser 5, the grid I5 is finally carried positive past the cut-off point with respect to cathode potential and plate current begins to flow in the tube I I. This current flows through the resistors 2l and 23 and is provided by the battery il which is preferably serially connected with resistors 2l and 23 and the output electrode I3 for the tube Il. The iiow of current through the resistors 2i and 23 produces a negative potential on the grid 2:9 of the cascade connected tube 25. The grid 29 of tube 25 connects with one terminal of resistor 2l and is maintained only slightly negative with respect to the cathode of tube 25 by a biasing source I9. The blocking potential induced on the grid 29 by the drop in potential through the resistors 2| and 23 cuts orf the plate current flowing through the tube 25 so that the grids St and it of the tubes 3l and respectively both become positive relative to their previous potential,

This is because of the decrease in plate current flow through tube 25 with a resulting decrease in the voltage drop through resistor 33 connected in the output circuit of the tube 25 and in series with the plate voltage supply source Si therefor which brings the point where the grids 39 and 45 connect with resistor more positive. A battery in series with the grids 3S and i5 of tubes 3l' and lll serves to apply a suitable biasing voltage. During such times as the grid electrode becomes positive due to a decrease in the voltage drop 'through resistor 33 the bias from battery is overcome and plate current ows through the tube il to actuate the relay il connected in the tube output circuit. Operation of relay [il will serve as a means to key a radio transmitter for example.

At the same time the grid ill-l oi tube 3l also becomes positive and plate current also hows in the tube 3l. The plate potential for tube 3l, however7 is furnished by the condenser 5 oi' which one terminal is connected with the plate electrode lil by way of conductor lid. The result is that when tube 3l is positively biased so as to draw plate current, the condenser 5 is discharged. When the condenser il is discharged, grid i5 of tube l i becomes negative, so that plate current ceases to flow in tube il and, since there is no longer any potential drop through the resistors 2l and connected in the output circuit, the grid 2:3 of tube becomes positive. This, in turn, permits plate current to now in the tube 25 and the resulting potential drop through resistor 33 is impressed on the grids 39 and i5 of tubes 3S and fil respectively, which cuts-oil the plate current and, in turn, restores the relay il to its initial position. At the same time the tube 3'] is rendered non-conducting so that the condenser 5 may be re-charged by the impinging light on the photocell l. The process is then repeated cyclically at a rate which is determined by the intensity of the light and the duration of the lights impingement.

It will be readily appreciated that ii the intensity of impinging light is great, that greater current will flow through the photocell, and consequently it will take less time to charge the condenser The result of this is that with great intensity of illumination on the photocell l, the relay 6l is actuated more times per unit interval of time than in the case where the intensity of the light is low. Thus, with low intensity of light considerable time must elapse before sufficient charge builds up on condenser 5 to actuate the grid l5 of the tube il and initiate the cycle of operation. It will be appree ciated, therefore, that this type of photocell amplifier can readily be used wherever it is de sired to have an indication of the integrated product or light intensity and time, since for each cycle of operation, the product of the intensity of illumination and the duration of its existence is the same.

In order that the discharging of the condenser 5 by the tube '3l shall not have a tendency to balance the fluctuations of the potential of the grid l5, a battery 'l is supplied which is connected between the two plate resisters 2l and 23 of the tube l l. To prevent this eiiect the current established through the plate 3S of the tube "il and the condenser o is furnished by the battery 'I through the resistance 23, such that the current flowing through the resistance 23 will make the grid 2Q of the tube .535 remain negative once the initial impulse has been given by the grid l5 of the tube il going positive. This will tend to cause the grid 2e of the tube 25 to remain negative until the discharge of the condenser 5, by the plate 58 of the tube 3l passing sufficient current, overcomes the positive condition that has already been given the grid lli of the tube l l.

Where it is desired to use the photocell and ampliiier, as an illumination integrating device, it is only necessary to replace the relay il-'l by a solenoid-operated counter, for example. This counter would then give an indication of the number of illumination-time units and by suitable calibration can be used as an illumination integrating meter. It is, of course, understood that when using the photocell and amplifier as a means of transmitting pictures and the like, that it is not necessary for the relay lll to key a radio transmitter. It might, for example, be used for communication over telegraph lines by sending out D.v C. impulses over the lines or it might be used on wire lines by having the relay il key a local oscillator generating a carrier tone.

Still other modiications of this invention may suggest themselves and may be made by those skilled. in the art without departure from the scope and spirit of the invention as defined in the appended claims.

What I claim is:

l. A facsimile system comprising in combination, a light translating device, an electric storage element connected with the light translating device and adapted to have stored therein an electrical charge of a magnitude varying in time in accordance with the brilliance of light innuencing the light translating element, a thermionic amplifying system connected with said storage element so that the energy output of said system is proportional to the time integrated value of light intensity iniiuencing said light translating device, means for controlling the output of said thermionic amplifying system so that the current flow therethrough takes place whenever the integrated value exceeds a predetermined value, and a relay connected in the output of the said thermionic amplifying system whereby said relay is actuated at a rate proportional to the integrated product of the time and brilliance of the light influencing said light translating element.

2. An indicating system comprising in combinae tion, a light translating device, an electric storage :fr

element connected with the light translating device and adapted to have stored therein an electrical charge of a magnitude varying in time in accordance with the brilliance of light influencing the light translating element, a therniionic amplifying device having an input and an output circuit, said input circuit being connected with the light translating element so that the output current ows from said device in accordance with the light intensity inuencing said light translating device and is proportional to the electrical charge stored in said storage device, a plurality of thermionic amplifying devices having parallelly connected input circuits connected with the output of said first named thermionic device, means for controlling the output of each of said last named thermionic devices in accordance with the current flow through the output circuit of the first of said thermionic devices, a connection between the output electrode of one of said last named thermionic devices and said electric storage element whereby said storage element servesy as a source of plate voltage for said therrnionic device, and an indicator connected in the output of the other of said last named thermionic devices whereby said indicator is actuated at a rate proportional to the integrated product of the time and brilliance of the light iniiuencing said light translating element.

3. An energy translating system which comprises a light translating element, an energy storing element connected with the output of said light translating device so that electrical energy is stored therein at a rate in accordance with the brilliance of light impinging upon said light translating device, a therrnionic amplifier device having its input circuit connected with the output of said light translating device and in parallel with said electric storage element so that the output current flowing through said device is controlled in accordance with the time integrated value of brilliance of the light impinging upon said light translating device, a second thermionic device having its input circuit con- D nected with the output circuit of said rst named therethrough varies inversely with the current flowing through said first named thermionic device, and a plurality of thermionic devices having parallelly connected input circuits connected with the output of said second named thermionic device so as to have the current iiowing therethrough controlled directli7 in proportion to the current fdowing through the first of said thermionic devices, a connection between the output electrode of one of said last named thermionic devices and said electric storage element so that said electric storage element serves as a source ci' plate current for said thermionic device and is discharged in accordance with the current owing therethrough, a load circuit connected with the second oi said last named thermionic device, and indicating means connected in said load circuit so that indications are produced upon said indicating means in accordance with the current flow in said load circuit and said indications are representative of the product of the brilliance of the light impinging upon said light translating element and the time period during which said light irnpinges upon said element.

RICHARD HOWLAND RANGER. 

